For the atomization of liquid products in the form of solutions or suspensions in connection with spray drying either a rotating atomizer wheel or nozzle atomization is used.
In general, rotating atomizer wheels have a higher yield per atomizing member than nozzle atomizers and are capable of atomizing liquids having a larger solids content and viscosity. As atomizer wheels possess, moreover, far larger tolerance to changes in solids content, viscosity and liquid flow, wheel atomization is usually preferred where possible.
However, powder products produced by use of wheel atomization present certain properties that make them less appropriate for some purposes. This applies for instance to the flowing properties of the powder, i.e. the ease with which the powder flows out of or down through an aperture. This property is important in several industrial operations and moreover in connection with the use of powder in certain beverage vending machines in which a determined portion of powder shall be quickly dosed to be mixed with water.
For this reason it is preferred, in some cases, to use powder produced by nozzle atomization irrespective of the above mentioned advantages of wheel atomization.
An unambiguous explanation of the difference of properties between the two types of products is difficult to give as the flowing properties are dependent on a series of factors, such as particle size distribution, the shape of the particles and the nature of their surface, and, as far as for instance fat milk powder is concerned, the amount of fat on the surface of the particles.